Introduce CurrencyConversion trait

Adds the `CurrencyConversion` trait to allow users to define custom
logic for converting fiat amounts into millisatoshis (msat).

This abstraction lays the groundwork for supporting Offers denominated
in fiat currencies, where conversion is inherently context-dependent.

Author: shaavan

lightning/src/offers/invoice_request.rs

@@ -76,8 +76,9 @@ use crate::offers::merkle::{
 };
 use crate::offers::nonce::Nonce;
 use crate::offers::offer::{
-	Amount, ExperimentalOfferTlvStream, ExperimentalOfferTlvStreamRef, Offer, OfferContents,
-	OfferId, OfferTlvStream, OfferTlvStreamRef, EXPERIMENTAL_OFFER_TYPES, OFFER_TYPES,
+	Amount, CurrencyCode, ExperimentalOfferTlvStream, ExperimentalOfferTlvStreamRef, Offer,
+	OfferContents, OfferId, OfferTlvStream, OfferTlvStreamRef, EXPERIMENTAL_OFFER_TYPES,
+	OFFER_TYPES,
 };
 use crate::offers::parse::{Bolt12ParseError, Bolt12SemanticError, ParsedMessage};
 use crate::offers::payer::{PayerContents, PayerTlvStream, PayerTlvStreamRef};
@@ -94,6 +95,7 @@ use bitcoin::constants::ChainHash;
 use bitcoin::network::Network;
 use bitcoin::secp256k1::schnorr::Signature;
 use bitcoin::secp256k1::{self, Keypair, PublicKey, Secp256k1};
+use core::ops::Deref;
 
 #[cfg(not(c_bindings))]
 use crate::offers::invoice::{DerivedSigningPubkey, ExplicitSigningPubkey, InvoiceBuilder};
@@ -573,6 +575,35 @@ impl AsRef<TaggedHash> for UnsignedInvoiceRequest {
 	}
 }
 
+/// A trait for converting fiat currencies into millisatoshi values.
+///
+/// This is used for handling conversions between fiat currencies and Bitcoin denominated in millisatoshis
+/// when working with Bolt12 invoice requests.
+///
+/// Implementors must provide a method to convert from a specified fiat currency (using ISO 4217 currency codes)
+/// to millisatoshis, handling any potential conversion errors.
+pub trait CurrencyConversion {
+	/// Converts a fiat currency specified by its ISO 4217 code to millisatoshis.
+	fn fiat_to_msats(&self, iso4217_code: CurrencyCode) -> Result<u64, Bolt12SemanticError>;
+}
+
+/// A default implementation of the `CurrencyConversion` trait that does not support any currency conversions.
+pub struct DefaultCurrencyConversion {}
+
+impl Deref for DefaultCurrencyConversion {
+	type Target = Self;
+
+	fn deref(&self) -> &Self::Target {
+		self
+	}
+}
+
+impl CurrencyConversion for DefaultCurrencyConversion {
+	fn fiat_to_msats(&self, _iso4217_code: CurrencyCode) -> Result<u64, Bolt12SemanticError> {
+		Err(Bolt12SemanticError::UnsupportedCurrency)
+	}
+}
+
 /// An `InvoiceRequest` is a request for a [`Bolt12Invoice`] formulated from an [`Offer`].
 ///
 /// An offer may provide choices such as quantity, amount, chain, features, etc. An invoice request